Revision 1866425

mod_dav_svn: Set Last-Modified response header for 'external' GET requests
(i.e. requests to URI's not under /!svn). This partially reverts r1724790
which removed it for all GET requests, for performance reasons (however, for
external requests the Last-Modified header is needed for certain use cases).

See this thread on dev@:
  https://lists.apache.org/thread.html/51f4f1fb5363c1d1c393f6ab769b12b9c7914af583fc333c652306d0@%3Cdev.subversion.apache.org%3E
  Date: 2019/09/02 14:20:16 GMT
  From: Johan Corveleyn
  To: Subversion dev list
  Subject: Re: Last-Modified HTTP header in GET responses
  Message-ID: <CAB84uBX4dNCsbaOn1Dzy0N3CB4rfC0_ZVCZ_TpVikt+p1zMGNg@mail.gmail.com>

Review by: brane

* subversion/mod_dav_svn/repos.c
  (get_last_modified): New function (reverted its removal in r1724790).
  (set_headers): Set Last-Modified response header for 'external' requests.

* subversion/tests/cmdline/mod_dav_svn_tests.py
  (last_modified_header): New test.